The present invention is directed to a device for imaging a printing form, having at least one laser diode bar, which includes a number of laser diodes whose image spots lie disjunctly in an image field, and having an array of microlenses positioned in the emission region of the light radiated by the laser diode bar, one microlens being assigned to each optical path of a laser diode. The present invention is also directed to a method for imaging a printing form, using at least one laser diode bar, which includes a number of independently addressable laser diodes whose image spots lie disjunctly in an image field and in whose emission region of the radiated light, an array of microlenses is positioned, one microlens being assigned to each optical path of a laser diode.
In printing-form imaging units or print units of printing presses, so-called direct-imaging print units, devices for imaging a printing form which have a plurality of imaging channels, in particular, those provided with solid-state lasers, semiconductor lasers or laser diodes, are often used simultaneously in order to efficiently reduce the time required to optically record an image on the two-dimensional surface of the printing form. If a redundancy-free imaging method is used, i.e., if the imaging channels are moved over the two-dimensional surface of the printing form in such a way that the point where each printing dot is to be set is passed exactly once by an imaging channel, then, given the use of an imaging device having n imaging channels, the imaging time needed for the entire surface to be recorded on is reduced to the (1/n)-times of the time. A further reduction can be attained just as efficiently when b imaging devices are used concurrently, which, in a redundancy-free manner, analogously to the procedure described above, expose sections of the printing form at a time. The imaging time for the entire surface to be recorded on is then reduced to the (1/b)-times of the time, to be precise, using b imaging devices having n imaging channels, to the (1/(bn))-times of the time.
The substantial reduction in the imaging time achieved by a redundancy-free parallelization is thus largely dependent on the number of imaging channels used or available (capable of being activated). The imaging speed scales linearly with the number of simultaneously used imaging channels. When a laser diode bar is used, in particular one constituted of an array of independently addressable laser diodes, as a light source in a device for imaging a printing form, in the context of a given, maximally attainable width that is conditional upon the technical production process, the number of imaging channels can only be increased by reducing the pitch between two adjacent laser diodes.
For the purpose of beam formation in devices for imaging printing forms which use laser diodes, in particular independently addressable laser diode bars (IAB), as a light source, microoptical components are generally provided in an optical system placed within the emission region of the laser diode bar, in order to collimate the radiated light in the direction of the slow axis of the diode lasers. In particular, microlens arrays, so-called SALA (slow-axis lens arrays) can be used individually or monolithically. Given the minimally attainable distance of the microlenses or of the SALA to the output facets of the laser diodes, the maximally permissible illumination of each individual microlens, as well as the particular divergence of the emergent laser radiation, the microlenses must have a minimum diameter and, consequently, a certain minimum distance from one another. This minimum distance represents a lower limit for the distance, i.e. the pitch, among the diode lasers and also for the number of diode lasers on the laser diode bar of a given width, when exactly one microlens is assigned to one laser diode under known methods heretofore. A device of the species for imaging a printing form using a semiconductor laser array, in whose emission region an array of microlenses is positioned, is discussed, for example, in U.S. Pat. No. 4,428,647.
With regard to the minimum distance among adjacent laser diodes, the following is thus to be noted. When beams diverging in the emission region of the laser diodes overlap already before or upstream from the microlenses or before the SALA, or when the divergent beams are wider than the microlenses (for example, greater in width than the output face) already at the microlenses or at the SALA, then the optical quality of the image spot produced by the optical system is seriously degraded. A significant portion of the radiation is sliced off. Often, an optical system for collimating or for reducing the divergence of the fast axis is positioned before the microlenses, so that the distances between the microlenses and the laser diodes cannot be reduced. Generally, it is not possible to reduce the angle of divergence of the beams emitted by the laser diode bar. Therefore, it is only possible to prevent the beams from being sliced off when a minimum distance is observed among adjacent microlenses.
In order for a number of imaging channels (regardless of whether they are arranged on one or a plurality of imaging devices) to pass the points on a two-dimensional printing-forme surface on which printing dots are to be set by image spots, in a redundancy-free manner, certain advance-feed rules must be observed for passing points imaged in a preceding step, to arrive at points imaged in a later step. These advance-feed rules must be strictly adhered to, particularly when, in an imaging step, n printing dots are to be placed by n imaging channels at points which do not lie densely on the printing form, i.e., whose spacing does not correspond to the minimal printing dot pitch p (typically 10 micrometers). To achieve a dense imaging, printing dots are placed between already recorded printing dots in a subsequent imaging step. This procedure is also known as interleaving. German Patent Application No. DE 100 31 915 A1, for example, characterizes an interleaving method for optically recording images on a printing form: At a given minimal printing dot pitch p, for a number of n imaging channels on a generated setting line, which are uniformly spaced apart and whose adjacent image spots on the printing form have a spacing a, which is a multiple of the minimal printing dot pitch p, a redundancy-free advance feed by the line motion (np) is ensured in the direction of the generated setting line, when the natural numbers n and (a/p) are prime.
It should be noted in this connection that the two-dimensional surface of the printing form to be recorded on is typically covered by the imaging channels quickly in a first direction and slowly in a second direction, which is linearly independent of and preferably normal to the first direction. In this context, the generated setting line generally does not lie in parallel to the fast first direction, but can be slanted at an angle, other than zero, from the slow second direction. This slant makes it possible to attain a printing dot pitch that is smaller by the factor of the cosine of the angle (projection). The generated setting line is preferably normal to the fast first direction. The image spots of the imaging channels can also be placed on the generated setting line by activation times which are delayed relatively to one another and between which the relative motion between the imaging device and the printing form is continued. Delayed activation times are useful, for example, for correcting geometric errors in the imaging device design and/or for precisely positioning the image spots (negligibly small displacement caused by delayed activation in comparison to the substantial relative motion).
From these explanations, it becomes immediately clear to one skilled in the art that a certain symmetry or a certain uniformity is necessary with respect to the pitch of adjacent laser diodes in the array of laser diodes on the laser diode bar, to make possible an interleave imaging method based on simple displacements (translational motions). For that reason, under the state of the art, laser diode bars for imaging devices feature a uniform distance among adjacent laser diodes.
The implementation of a redundancy-free interleave method in accordance with the German Patent Application No. 100 31 915 A1 is critically dependent on n imaging channels being available, thus capable of being activated, at a uniform pitch on a generated setting line. As a strategy to follow in the event an imaging channel fails or does not operate properly, it proposes using the largest, still cohesive section of the imaging channels at a uniform distance in order to avoid streaks on the printing form where the surface is not recorded on, and to ensure an invariably good imaging quality. It is clear that to implement a redundancy-free interleaving method in accordance with this document, one must select a number of imaging channels of the still cohesive section that is prime to the multiple of the pitch (a/p). Following this strategy, any further imaging channels experiencing failure or improper operation result in only very short sections of the original n parallel imaging channels remaining. Consequently, the imaging time increases considerably with the decrease in any still remaining parallelization. For example, in the least favorable instance, for every imaging channel that fails in the middle of the largest cohesive section on the generated straight line, the imaging time increases to twice the parallelized imaging time, thus for a plurality of failures, to a multiplicity thereof. This is completely unacceptable in practical applications.
Especially critical when laser diode bars are used in imaging devices is generally the failure or improper operation of a laser diode when exactly one laser diode is assigned to each imaging channel. This is because restoring the original reliable operation requires replacing the entire laser diode bar. This is not practical for economic reasons alone, because the other laser diodes on the bar are generally still in proper service condition, so that the laser diode bar has not completely lost its ability to function.
U.S. Pat. No. 6,181,362 B1 proposes assigning two laser diodes on one laser diode bar to each imaging channel. To record on a printing form, one laser diode is used for each imaging channel. In the event that the first laser diode in one imaging channel fails, the second laser diode is used in its place. Alternatively, U.S. Pat. No. 6,252,622 B1 proposes assigning each imaging channel a primary laser diode on a first laser diode bar and a secondary laser diode on a second laser diode bar. To record on a printing form, one laser diode of one of the two laser diode bars is used per imaging channel. In the event the primary laser diode on the first laser diode bar in one imaging channel fails, the secondary laser diode on the second laser diode bar is used in its place. One drawback of these proposed approaches is, inter alia, that, when working with a defined emission characteristic of the laser diodes on one laser diode bar that determines the minimal spacing between adjacent laser diodes, the width of the laser diode bar must be increased, or a second laser diode bar must be positioned more precisely in relation to a first laser diode bar. The result is that the overall height of the device for imaging a printing form is increased.